This invention relates to the preparation of silazane polymers. These polymers are useful as chemical intermediates to synthesize organosilicon compounds. They are also useful, when fired at high temperatures, to form silicon carbide and silicon carbide containing ceramic materials.
What is disclosed herein is a novel process to obtain novel silazane polymers which consists of contacting and reacting chlorine-containing disilanes with disilazanes in an inert, essentially anhydrous atmosphere while distilling volatile by-products.
As is well-known in the art, halosilane monomers will react with ammonia and most organic compounds containing a primary or secondary amino group to give a variety of silazanes. For example, the reaction of trimethylchlorosilane and ammonia produces hexamethyldisilazane, a silazane monomer, while dimethyldichlorosilane and ammonia produce dimethylcyclic silazanes. These reactions probably constitute the majority of commercial uses of the silazane chemistry.
Silazanes in general have been academic curiosities for many years and a variety of such silazanes, including monomers, oligomers, cyclics and even low molecular weight resins and linear polymers have been prepared by a variety of methods. For example, L. W. Breed et al., in the Journal of Organic Chemistry, 27, 1114(1962) report the formation of silazanes from the polymerization of sterically hindered silazane oligomers, while in the Journal of Polymer Science, A 2 45(1964), cyclic trimer and tetramer silazanes are reported to be thermally cracked using catalysts to give linear polymers.
In contrast, fluids, rubbery polymers and resins prepared from CH.sub.3 SiCl.sub.3, (CH.sub.3).sub.2 SiCl.sub.2 and excess ammonia have been reported by Kruger et al. in the Journal of Polymer Science, A 2 3179(1964) and Redl, Silazane Polymer, ARPA-19, Advanced Research Projects Agency, October, 1965.
The patent literature also contains disclosures of the preparation of silazanes. Cheronis, in U.S. Pat. No. 2,564,674 discloses the preparation of low molecular weight linear silazane polymers by the reaction of halosilanes with excess ammonia in a solvent solution. Bausma, et al., in U.S. Pat. No. 3,809,713 discloses a similar reaction scheme with the added modification of removing the by-produced solid ammonium halide using ethylene diamine.
More recently, Verbeek, et al., in U.S. Pat. No. 3,853,567 and U.S. Pat. No. 3,892,583 disclosed that mixtures of CH.sub.3 SiCl.sub.3 and (CH.sub.3).sub.2 SiCl.sub.2 can be treated with ammonia or organoamines to form materials that can be pyrolyzed to yield SiC/Si.sub.3 N.sub.4 ceramics.
As should be recognized by those skilled in the art, the present invention differs in at least one respect from all of the above art in that the present invention is based on chlorine-containing disilanes as opposed to the use of chlorine-containing monosilanes.
In another segment of the prior art, the use of disilanes in the preparation of silazane polymers has been limited to the formation of relatively low molecular weight materials. In one example, Wannagat et al., Ang. Chem. 75(7) 345(1963), reported the reaction of tetramethyldichlorodisilane with gaseous ammonia to give a six-membered cyclic silazane, {(CH.sub.3).sub.2 SiSi(CH.sub.3).sub.2 NH}.sub.2, rather than the expected linear silazane polymer and Hengge et al., Montash. Chem. 101I(2)325(1970), prepared dimethylamino substituted mixtures of disilanes from dimethylamine and the chlorine-containing disilane mixture obtained from the Direct Process for the preparation of chlorosilanes.
What has been newly discovered is the coreaction between chlorine-containing disilanes and disilazanes to give high molecular weight silazane polymers.